1. Field of the Invention
The present invention relates generally to CDMA (code division multiple access) communication systems, and more specifically to a downlink power control method and a system using the same.
2. Description of the Related Art
A transmit power control scheme for downlink (base-to-mobile) channels of CDMA communication systems is described in “3GPP RAN (3rd Generation Partnership Project Radio Access Network) 25.214 v1.3.1”. According to this document, each mobile station constantly monitors its downlink channel and determines its signal-to-interference ratio (SIR). The mobile station compares the SIR value with a prescribed target value and transmits a TPC (transmit power control) command signal through an uplink channel, requesting the base station to increase or decrease the power level of the downlink channel. The power level of a downlink channel is varied by a predetermined incremental unit for each TPC command signal. Power control will be repeatedly performed if the base station repeatedly receives TPC command signals until the upper or lower limit of a power control range is reached. The minimum power control limit is determined in consideration of the fact that, when a power decrease takes place in a downlink channel of excellent signal quality, the signal quality at the reduced level still allows the base station to respond to a possible degradation which may subsequently occur due to a sudden movement of the mobile station. The maximum power control limit of the base station is determined by taking account of interference between mobile stations which would be caused by possible racing conditions in which they compete for power increase. The number of channels allocated to the base station is also a determining factor of the maximum limit of the control range.
However, one shortcoming of the prior art scheme is that, since power control is effected in a specified range that prevents the base station to transmit its power at a level below the minimum power control limit, those mobile stations that are located near the base station would receive power more than what they actually need for their downlink channels. As a result, useful energy resource of a base station is wasted. Another shortcoming of the prior art is that, due to the presence of the upper limit, those mobile stations that are located far off the base station would receive power less than what they actually need for their downlink channels even when the transmit power level of the base station still has a sufficient amount of allowance with respect to its maximum power control limit.